US President George W Bush has signed a controversial bill into law that will pay for a 700-mile fence (1,125km) fence along the border with Mexico.
Mr Bush signed the law in Arizona, where illegal immigration is a concern
The barrier, equipped with hi-tech surveillance equipment, will be built in areas where many illegal immigrants cross over into the United States.
Mr Bush has also suggested a temporary guest worker programme to allow immigrants to work legally in the US.
But Republican congressmen have argued against the idea.
They said it would be an amnesty that would give illegal immigrants a route to citizenship.
Under the new law, around $1.2bn will be spent over the coming year on the border, which will be built on remote and inhospitable border areas in the south west of the country.
Speaking at the signing of the bill in Arizona, Mr Bush said the fence alone was not enough to curb illegal immigration.
"The funds that Congress has appropriated are critical for our efforts to secure this border and enforce our laws, yet we must also recognise that enforcement alone is not going to work," he said.
"We need comprehensive reform that provides a legal way for people to work here on a temporary basis."
There are an estimated 12 million illegal immigrants living in the US.
An estimated 1.2 million illegal immigrants were arrested last year trying to cross into the US along the border states of Texas, New Mexico, Arizona and California.
Supporters of the fence have said it will be a crucial tool to fight illegal immigration - which is expected to be a key issue in November's mid-term elections.
But critics of the scheme say it will force illegal immigrants to cross somewhere else.
Mexico has also said the fence will badly affect relations with the US.
The outgoing Mexican President, Vicente Fox, has called the fence shameful, and compared it to the Berlin Wall.